Ultrashort optical pulses are of importance for many technological and scientific applications. One known technique for producing such pulses is generally referred to as mode-locking. See, for instance, the chapter by C. V. Shank in "Ultrashort Laser Pulses and Applications", W. Kaiser, editor, Springer Verlag 1988, pp. 5-34. In particular, mode-locked semiconductor lasers are likely to become increasingly important for, e.g., high speed optical communication systems, very high resolution optical time domain reflectometers, electro-optic sampling systems, and as clock signal source for high speed electronic circuits. Desirably the ultrashort pulses are "transform-limited", or nearly transform-limited, since transform-limited pulses require, for a given pulse length, the least bandwidth, and are likely to be able to propagate greater distance in optical fiber without suffering unacceptable distortion.
Although most mode-locked semiconductor lasers to date utilized external cavities, monolithic mode-locked semiconductor lasers are known. See R. S. Tucker et al., Electronics Letters, Vol. 25(10), pp. 621-622 (1989); and P. A. Morton et al., Applied Physics Letters, Vol. 56(2), pp. 111-113, (1990). Such monolithic lasers have obvious advantages in terms of, e.g., stability and size over non-monolithic ones. However, these prior art monolithic lasers have not succeeded in producing nearly (i.e., within a factor of two) transform-limited optical pulses.
The prior art knows many examples of the generation of ultrashort laser pulses by "colliding pulse mode-locked" (CPM) dye lasers. See, for instance, the above cited article by C. V. Shank. The CPM technique, when used with dye lasers, has succeeded in generating femtosecond transform-limited pulses. The central aspect of the CPM technique is the utilization of the interaction (frequently referred to as the "collision") of two counter-propagating optical pulses in an optical cavity to enhance the effectiveness of the "saturable absorber" commonly present in mode-locked lasers.
In view of the advantages associated with ultrashort optical pulses, a stable, efficient, small size solid state source, capable of producing transform-limited (or nearly transform-limited) pulses, would be of substantial importance. This application discloses such a source.